1. Field of the Invention
This invention relates to a method for synthesizing trioxane, characterized by heating and reacting formaldehyde in the presence of a heteropolyacid.
2. Description of the Prior Art
This invention relates to a method for synthesizing trioxane from formaldehyde in the presence of a heteropolyacid.
Trioxane is a cyclic trimer of formaldehyde and is used mainly as a starting material for producing polyoxymethylene. In order to obtain polyoxymethylene having a satisfactory molecular weight, very pure trioxane is required and it is necessary that its contents in particular, of the so-called by-products such as water, formic acid and methanol which become chain transfer agents, methylal, methyl formate, and polyoxymethylenedimethoxide with a low molecular weight, and the like should be low.
Many researches have been done on a method for synthesizing trioxane satissfying these requirement, though a usual production method consists of obtaining trioxane by heating formaldehyde in the presence of an acid catalyst, for example, sulfuric acid, phosphoric acid, boric acid, benzenesulfonic acid, toluenesulfonic acid, an acidic ion-exchange resin, or the like, or in the presence of a solid acid catalyst such as aluminum sulfate, silica or the like. Among the above-mentioned catalysts, sulfuric acid is most generally used because the reaction rate is fast when it is used, and it is easily available. However the sulfuric acid method have involved several practical problems to be solved. For example, when the formaldehyde concentration exceeds 60% by weight, paraformaldehyde is formed, and when the sulfuric acid concentration is 8% by weight or higher, by-products such as formic acid, methyl formate and the like are formed so that the yield of trioxane is lowered. Further, when the sulfuric acid concentration is 10% by weight or higher, many by-products are formed, therefore it has been proposed to effect the reaction by adding a dispersing agent such as di-2-ethylhexyl phthalate, however the addition of the third component makes post-treatment and the like troublesome and hence is not thought to be a suitable method. As mentioned above, conventional methods have had many disadvantages for application on an industrial scale, for example, (1) many by-products are formed, (2 ) scales of paraformaldehyde tend to be formed in th reactor or on the wall of the distilling column, (3) the reactor and the wall of the distilling column are corroded.